Variable displacement engines are known in which the intake valves of selected cylinders are mechanically disabled and the remaining cylinders continue to operate. This results in improved fuel economy because the remaining cylinders operate with a wider throttle angle than is the case when all cylinders are operating.
The inventors herein have recognized numerous problems with the prior approach to variable displacement engines. One particular problem is that complex mechanical devices are required to deactivate the intake valves. Another problem is that rough idle conditions may result when in the variable displacement mode, particularly for engines in which the ignition firing order is not limited to each engine bank such as a V-8 engine. It has therefore been common not to operate such engines in idle speed modes.
Another approach has been to disable injected fuel to a selected number of cylinders. Such a system is described in U.S. Pat. No. 6,023,929, as well as in JP-A-55 029002 and JP-A-55 59549.
However, the inventors herein have also recognized that such systems do not achieve the full benefit possible from such a system. In particular, the inventors herein have found, as described below, that engines can obtain further fuel efficiency benefits than those provided by the above methods.